Buy-all Images In 2 Different Sub Galleries

I'm trying to figure out how to offer customers to purchase all images from 2 sub galleries either by download or a mailed USB. So for example the cost is $100. for both galleries, how do I set it up where they need to choose the 2 sub galleries? Can a buy all be placed into a collection and require them to pick the 2 sub galleries?

I'd love to see a way to do a buy all on a sub gallery. For example a school where each kid is in a sub-gallery. Then the parent can buy all the images of their kid. Same with what I want to do. Each kid on a particular event is their own sub gallery and need a way for a buy all on that sub gallery.

I've tried a few times but can't seem to get it to work, or maybe I did but it looks too confusing oh how to select both galleries as part of the collection.
If I was able to do a buy all just as a product and have a quantity discount that would have worked much easier.